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I am a newbie trying to detect NOAA 15, 18, and 19 satellites, which have downlink frequencies between 137 MHz and 138 MHz. I am using an SDR dongle and attached a photo of it to this post.

Of course, I will be constructing an antenna specified for the 137 MHz later. However, I wanted to see if I could observe anything with this configuration. Observing the 137-138 MHz band, I observed an interesting pattern, which seems like a sinc function as if some square signal is multiplied with a sinusoidal signal. Here is a photo of my observation below.enter image description here

Now here are my questions about these:

-> What this frequency might be about? I can detect it inside my house and outside as well. I looked it up and could not find a corresponding signal. Is this something caused by my setup? How can I observe NOAA satellites with similar frequencies?

-> There is also a peak that I could not decode at 137.5 MHz. Seemed like an FM signal to me but not sure. Do you have any idea on that as well?

Any idea would help if you have an opinion on what these signals might be about. Take care in the quarantine times.

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    $\begingroup$ That looks like you're strongly overdriving your receiver. Drastically reduce gain. This signal might not come from 137.5 MHz at all but be mixed there by the nonlinear effect of overdriving your amplifiers. $\endgroup$ – Marcus Müller Aug 28 at 9:10
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    $\begingroup$ Do you mean too high gain of an LNA inside the sdr caused this sinc type shape? Is it because it is amplified so much that it becomes saturated and resulting in a square-like distribution? Sorry for the number of questions by the way and thank you for your attention. I will try reducing the LNA gain in the software but I can receive and listen to the FM radios nicely. Now, I am thinking that reducing the gain might make it harder to observe the NOAA satellite signals. On the other hand, noises will be lower as well. In conlcusion, I will try it again by reducing the gain, thanks a lot! $\endgroup$ – TEQATAR Aug 28 at 10:34
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    $\begingroup$ really, reduce gain. Just because you can listen to something else nicely with that gain doesn't mean it's appropriate here. Also, FM is robust against nonlinearities by being FM. $\endgroup$ – Marcus Müller Aug 28 at 11:28
  • $\begingroup$ Welcome to this site! Please let us know if @MarcusMüller 's suggestion helped you. $\endgroup$ – Mike Waters Aug 28 at 14:39
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    $\begingroup$ @MikeWaters Hello! Yep, it is logically explanatory to me and I accepted it as an answer. Thanks a lot for your ideas on the topic and have a nice day! $\endgroup$ – TEQATAR Sep 1 at 18:31
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That looks like you're strongly overdriving your receiver. Drastically reduce gain. This signal might not come from 137.5 MHz at all but be mixed there by the nonlinear effect of overdriving your amplifiers.

The fact you can receive FM (a different signal at a different frequency) doesn't mean the gain is appropriate for this signal. Also, FM, being FM, is relatively robust against nonlinearity (i.e. distortion by too much gain bringing your receiver to the edges of its value range).

Less gain doesn't mean less reception — imagine this: you use a hearing aid. Somebody screams into your ear. Is it wise to have your hearing aid's gain at a high level?

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